Drive systems of this kind enable the transmission ratio of the gears to be varied continuously over a wide range, both upwards and downwards, and also make optimal steering and driving conditions possible when used in a vehicle, both when driving straight ahead and when turning.
A drive system of the above-mentioned kind is described and illustrated in German Offenlegungsschrift No. 21 52 716. In this design the control pressures acting on the servo-regulating devices are set by adjustable control pressure indicating valves that can be adjusted by a control means, which in the case of a track-laying or caterpillar vehicle is a speed control means and/or a steering means.
However, the known design suffers from the disadvantage that the actual pump deliveries only theoretically correspond to the desired amounts. In practice it is found that the pump deliveries controlling the speed of the gear concerned or the speed of a vehicle differ from the desired values. This defect arises from sources of error in the characteristics of the determination of the delivery amounts, which are mainly determined by the movement of the servo-regulator and cannot be wholly avoided by acceptable means. In particular what are involved are departures from intended values arising from manufacturing tolerances, e.g. differences in the tensions of the springs used and differences in dimensions, particularly of the control edges of the indicating valves in the servo-regulators, differences in frictional losses of the moving parts of the gear, especially of the servo-regulators, different deliveries caused by manufacturing tolerances and/or leakage losses in the pumps and the like. Differences in the intake volumes of the hydraulic motors arising from manufacturing tolerances and/or leakage losses also contribute to the above-mentioned sources of error.
The above-mentioned defect is made particularly apparent in the case of the known design through the output speeds of the gears not corresponding exactly to the desired values. This is not so important when driving, since the resulting driving direction can be corrected by the steering means. Problems arise when driving if the steering means can be fixed in a position that corresponds to movement straight ahead, since when the steering means is in this position the vehicle does not move precisely straight ahead.
The defect in the known design becomes particularly apparent on starting and shows up in the case of a vehicle by a jerk to one side, since the driving means of the vehicle are acted upon in different ways.